Recent years have seen a proliferation of digital cameras, such as digital still cameras and digital video cameras. There are many consumer applications that require digital image capture. These applications can include, for example, still motion imaging, full motion imaging, security and surveillance applications, and video-conference applications. An important component in digital still cameras and digital video cameras is the image sensor integrated circuit. Image sensor integrated circuits employ an array of pixels (e.g., a 640×480 pixel array) for detecting photons of light and for converting the photons into an electrical signal (e.g. voltage).
Most image sensor integrated circuits are manufactured by utilizing a complimentary metal oxide semiconductor (CMOS) process. Unfortunately, as with any manufacturing process, image sensor parts have defective pixels. Customers of these parts do not wish to have any obvious defective pixels.
One approach to address defective pixels is to establish strict blemish specifications. For example, vendors of image sensor integrated circuits establish blemish specifications that prevent the shipment of sensors with more than a predetermined number of defects, with defects in certain locations, or with certain types of defects. Unfortunately, the blemish specifications result in lower yields and lower volumes for the manufacturer of the image sensors and higher costs for the customers.
A second approach to address the defective pixel problem is to use some type of defective pixel correction scheme. There are some CCD image sensors that have built-in hardware for use in correcting defective pixels. Unfortunately, these implementations require complex hardware components that occupy a large physical space, thereby making the scheme expensive to implement. Accordingly, these expensive and cumbersome solutions are relegated to only a few applications (e.g., high-end and low-volume applications).
Another hardware approach is described in a semiconductor technical data entitled, “Digital Image Sensor—SCM20014,” available from Motorola, Inc. Motorola's digital image sensor decides that pixels are defective independently from frame to frame and then corrects the defective pixels by replacing them with a varying value derived from neighboring pixels for that particular frame. Unfortunately, as described in greater detail hereinafter, this approach utilizes a detection and correction technique that can vary from frame to frame, thereby leading to undesirable image artifacts.
Moreover, some have proposed the use of software-based defective pixel correction techniques. Unfortunately, these prior art approaches require a processor with sufficient processing power to execute the software algorithms that perform the defective pixel correction. As can be appreciated, these software techniques are not available to products that do not have a processor. Furthermore, even for those products that have a processor, the software techniques consume large amounts of processor bandwidth, which may not be acceptable to other components that also require the processor bandwidth. As can be appreciated, the requirement of these software-based techniques for a processor increases the cost of such a product so that such an approach is also relegated to high-end image capture devices.
Some have proposed the use of a personal computer (PC) that can be coupled to a digital capture device (e.g., a digital camera) to execute the correction techniques. In such an approach, the digital capture device first sends the image with the defective pixels to the PC, and then the PC executes the defective pixel correction schemes to correct the image. An advantage of this approach is that the hardware required in the digital capture device can be simplified since the processing power of the PC is harnessed for executing the defective pixel correction software. Unfortunately, most PC connectable capture devices perform data compression on the image in order to transfer the data to the PC through a link (e.g., a USB cable). When compression is applied to the image, there is a high likelihood that the defects spread out and the appearance of the image becomes worse, making correction at the PC more difficult. Consequently, it is desirable to perform defective pixel correction in the sensor IC.
Furthermore, these prior art approaches provide 1) inconsistent defective pixel detection, and 2) inconsistent defective pixel correction, thereby causing undesirable artifacts in the resulting image.
One disadvantage of these prior art approaches is that the defective pixel detection may determine that a particular pixel is defective in one frame and not defective in another frame (hereinafter referred to as inconsistent defective pixel detection). As can be appreciated, inconsistent defective pixel detection provides inconsistent results from frame to frame, thereby leading to a loss of resolution at a pixel location. For example, in a first frame a particular pixel may be determined to be a defective pixel that is replaced with another value. In another frame, the same pixel may be determined to be a non-defective pixel that retains its pixel value. When this approach is applied in a digital video capture device, the inconsistent detection can cause artifacts in the resulting video, such as a blinking spot. Similarly, when the approach is applied in a digital still camera, the inconsistent detection can cause artifacts (e.g., a bright spot or spots) in the resulting image. Consequently, it would be desirable for there to be a defection pixel correction mechanism that consistently detects defective pixels from frame to frame.
Another disadvantage of this approach is that the defective pixel correction may replace the defective pixel value with different replacement choices from frame to frame (hereinafter referred to as inconsistent defective pixel correction). For example, in a first frame a pixel that is determined to be defective is replaced with a first value. In a second frame the same pixel that is defective may be replaced with a second value. In a digital still image, inconsistent replacement values for adjacent pixels may not be critical. However, for digital video, an inconsistent replacement value from frame to frame is very noticeable to the human eye. For example, a replacement value that varies for a defective pixel from frame to frame may appear to the human eye as a blinking spot in the video.
Based on the foregoing, there remains a need for a defective pixel correction method and system for image sensors that overcomes the disadvantages set forth previously.